As is known, in supersonic aircraft, the compressors for the engines are incapable of accepting supersonic airflow. Consequently, the engine air inlet ducts for such engines must be designed to produce shock waves which reduce the velocity of the entering air to a subsonic level which can be accepted by the compressors. The shock waves comprise areas or boundaries at the intake where the velocity of the entering air decreases abruptly while its pressure increases. Successive decreases in velocity and increases in pressure bring the air down to a subsonic velocity where it can be accepted by the engine compressors.
Two-dimensional external compression supersonic inlets of this type typically consist of an external ramp system followed by a subsonic diffuser duct which reduces the velocity of the captured airstream from the free stream Mach number to the desired engine face Mach number. The external ramp system, which compresses the flow to a Mach number slightly above 1.0, is usually provided with sideplates to prevent spillage of the compressed airflow over the edges of the ramps. Compression from the low supersonic Mach number at the end of the external ramp system to subsonic flow is obtained through a normal shock (i.e., normal to the ramp and side walls) which typically is located just upstream of the lower lip of the subsonic diffuser duct. For off-design operation of the supersonic aircraft, it may be necessary to reduce the amount of flow captured by the inlet without changing the position of the external ramp system. This is accomplished by advancing the normal shock forward on the ramp system. Advancing the normal shock forward increases the spillage of subsonic flow over the cowl lip. The amount of spillage (i.e., stability margin) available from the forward movement of the normal shock is limited by a flow instability phenomenon which causes the normal shock to oscillate severely. Adequate stability margin, therefore, may not be available.
A weakness of prior art two-dimensional supersonic inlets is associated with their sideplates and is characterized by a high degree of sensitivity to side flow, especially during supersonic flight conditions. Cut-back sideplates have been used in the past to alleviate this problem. Such cut-back sideplates tend to increase the stability margin by providing increased flow area through which subsonic spillage can occur. Unfortunately, however, there is a penalty associated with this very simple solution to the inlet aerodynamic problems. That is, part of the flow compressed by the external ramps is spilled over the cut-back sideplates, thereby reducing the efficiency of the inlet system (i.e., increasing the size and drag and reducing the total pressure recovery of the inlet).